Off-photometric-range indicator for exposure meter

ABSTRACT

An off-photometric-range indicating device for photometric circuits for cameras visually indicates to a photographer when the brightness of an object to be photographed is off the set photometric range of the photometric circuit. The indicating device includes a detection circuit to detect the brightness of the object and to produce an output signal when the detected value of the brightness is out of the photometric range of the photometric circuit, a switching circuit connected to the detection circuit and operated by said output signal, and a circuit connected to the switching circuit and actuated by the operation of the switching circuit to scale out the indicator, which may be visually observed by the photographer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an off-photometric-range indicator in exposuremeters for cameras whereby when the brightness of the object to bephotographed is not consistent with the set photometric range of thephotometric circuit such condition is visually indicated to thephotographer.

2. Description of the Prior Art

Among the known techniques relating to such indicating devices is amethod according to which when the luminous intensity on the lightreceiving surface of a receptor element becomes higher or lower than acertain preset absolute value, such occurrence is indicated by operationof a luminous element such as luminous diode, as for instance proposedin U.S. Pat. No. 3,810,208. This method, as disclosed in said patent,has the advantage that the structure of the device is simplified as nocomplicated mechanical arrangement is needed; but, on the other hand, ithas a drawback in that extra space is required in which to incorporate aluminous element such as luminous diode which is indispensable in thismethod, and in some cases, there is further necessitated an extraoptical system for guiding the light beams from the luminous elementinto the view finder. Also, as the luminous element, such as luminousdiode, consumes relatively high quantities of power as compared withother circuit parts, the battery is quickly exhausted. This becomes aserious problem, particularly when the camera is kept in storage for along time. Usually, when a camera is not used, it is placed in a darkplace where the ambient light intensity is outside the photometric rangeof the photometric circuit, so that if the power switch of the warningdevice is kept on, the luminous element in the warning device remainslighted to consume power wastefully, and this could result in exhaustionof the battery during storage of the camera. In the case of a TTL(through-the-lens) photometric camera, the aforementioned difficultycould occur even if the camera is not placed in a dark place, becausethe light to the receptor element is shut off if the lens cap is leftmounted.

Further, conventional exposure indicators often give false indicationsto misguide the photographer when the light intensity of the object isoutside the set photometric range.

SUMMARY OF THE INVENTION

I have conceived an improved off-photometric-range indicator device bywhich I am able to provide a photometric range indication by using theexposure indicator as the off-photometric-range indicating element so asto give off-range indications with the scaleout of the indicator.

According to this invention, the off-photometric-range indicating deviceincludes a detection circuit connected to a photometric circuit, aswitching circuit connected to the detection circuit and a furthercircuit connected to the switching circuit to scale out the indicator.The detection circuit detects the brightness of the object and producesan output signal when the detected value of the brightness is outsidethe photometric range, the switching circuit operates when receivingsaid output signal and the other circuit actuates by the operation ofthe switching circuit to scale out the indicator.

There has thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood, and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto. Thoseskilled in the art will appreciate that the conception upon which thisdisclosure is based may readily be utilized as a basis for the designingof other structures for carrying out the several purposes of theinvention. It is important, therefore, that the claims be regarded asincluding such equivalent constructions as do not depart from the spiritand scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention have been chosen for purposes ofillustration and description, and are shown in the accompanyingdrawings, forming a part of the specification wherein:

FIG. 1 is a circuit diagram showing a first embodiment of the presentinvention;

FIG. 2(a) illustrates the exposure scale in the visual field of theviewfinder with a pointer as positioned when the object brightness iswithin the photometric range;

FIG. 2(b) is similar to FIG. 2(a) but shows the pointer positioned toindicate that the object brightness is outside the photometric range;and

FIG. 3 is a circuit diagram showing a second embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference tothe preferred embodiments thereof.

Referring to FIG. 1, there is shown a first embodiment of the presentdevice as it was adapted in an exposure indicating circuit for anelectric shutter. According to this embodiment, the portion A is a knownexposure indication circuit for an electric shutter, and the portion Billustrates the novel circuitry according to the present invention.

I will first describe the circuit arrangement shown in the portion Awherein a photometric circuit is constituted by a photoconductiveelement 1 such as Cds cell and a diode 2 which are connected in seriesto each other so that an output voltage proportional to the logarithm ofthe brightness of the object to be photographed is produced at thejunction therebetween. Also, an exposure factor introduction circuit isconstituted by a variable resistance 5 with its resistivity changingcorresponding to the setting values of aperture and film sensitivity(when such aperture and film sensitivity are considered as exposurefactors) and a diode 6 connected in series to the variable resistance sothat an output voltage related to the aperture and film sensitivity isproduced at the junction therebetween.

The output of the photometric circuit 1, 2 is applied to one terminal ofa voltmeter 9, which serves as an exposure indicator, through anemitter-follower circuit formed by a transistor 3 and a resistance 4,while the output of the exposure factor introduction circuit 5, 6 is fedto the other terminal of the voltmeter 9 through an emitter-followercircuit formed by a transistor 7 and a resistance 8. The voltmeter isdesigned to give a proper exposure indication according to thedifference between both outputs.

The shutter control circuit 10 is adapted to receive both the output ofsaid photometric circuit 1, 2 and the output of the exposure factorintroduction circuit 5, 6 so as to control the shutter speedcorresponding to the difference between both outputs.

Referring now to the portion B of FIG. 1, a reference voltage generatingcircuit is constituted by a resistance 11 and a diode 12 connected inseries so that there is produced at the junction thereof an outputvoltage which is a divided part of the voltage of the power source 20,and which is equal to the output voltage of the photometric circuit 1, 2when the object brightness is at the lower limit of the photometricrange of the exposure indicator.

The transistors 13, 14, 15 and 16 and a resistance 19 constitute adifferential amplifier to which the output of the photometric circuit 1,2 and the output of the reference voltage generating circuit 11, 12constitute an input. A detection circuit is also constituted by thereference voltage generating circuit and the differential amplifier. Aresistance 18 is connected between the power source 20 and the otherterminal of the voltmeter 9 through a switching transistor 17 which isoperated by the output of the differential amplifier 13, 14, 15, 16, 19.Resistivity of this resistance 18 is so set that when the switchingtransistor 17 becomes conductive to connect the resistances 18 and 8 tothe power source 20, a voltage acting to scale over the pointer of thevoltmeter will be produced at the junction of both resistances; that isto say, at the other terminal of the voltmeter 9. Thus, resistance 18constitutes a circuit for scaling out the voltmeter 9.

Being constructed as substantially described above, the present deviceoperates as follows.

When the object brightness is sufficiently high and stays within the setphotometric range, the output voltage of the photometric circuit 1, 2 ishigher than the output voltage of the reference voltage generatingcircuit 11, 12, and hence the collector current of the transistor 14 isgreater than that of the transistor 13, so that the transistor 15 issaturated to make the transistor 17 non-conductive. Therefore, theoperation of the voltmeter 9 is not affected by the resistance 18 andproper shutter speed is indicated by the pointer 9a of the voltmeter 9on the shutter scale 21 in the viewfinder of the camera, as shown inFIG. 2(a).

When the object brightness is lower than the lower limit of thephotometric range, the output voltage of the photometric circuit 1, 2becomes lower than the output voltage of the reference voltagegenerating circuit 11, 12, and hence the collector current of thetransistor 13 becomes greater than that of the transistor 14, so thatthe transistor 15 becomes active or non-conductive while making thetransistor 17 conductive. As a result of conduction of this switchingtransistor 17, a divided portion of the power source voltage is appliedto the other end of the voltmeter 9 through the resistances 8 and 18, sothat the pointer 9a of the voltmeter 9 scales out without giving anyfalse indication, and is positioned outside the shutter speed scale 21,as shown in FIG. 2(b). The voltmeter 9 in this embodiment is so arrangedthat when the power switch 22 in FIG. 1 is opened to cut off currentsupply to the circuit, the pointer 9a points "1/15" at the middle of thescale, so that when the object brightness is outside the photometricrange, such can be indicated by scale-out of said pointer 9a.

To cite an actual instance, in case the photometric range is above EV 1when using an ASA-100 film, if ASA-100 exposure and F 1.4 setting ismade on the camera, the optimum shutter speed under the brightness of EV1 (which is at the lower threshold value of the photometric range) isone second. Therefore, if the object brightness is gradually reducedfrom a value above EV 1 under this condition, the pointer moves slowlydownwards while indicating the proper shutter speed for each brightnessuntil it points to 1 second in FIG. 2(a), and at the moment when theobject brightness becomes lower than EV 1, the pointer scales overrapidly to take the position shown in FIG. 2(b). From this, thephotographer can learn that the object brightness has passed out of thephotometric range.

Referring now to FIG. 3, there is shown a second embodiment of thepresent invention wherein the circuit elements bearing the numeralscorresponding to those in FIG. 1, operate in the same manner as in thefirst embodiment, wherefore only those parts different from those of thefirst embodiment will be explained in the following description.

I provide a switching transistor 25 operated by a differential amplifier(constituted by the elements 13, 14, 15, 16 and 19), with the base ofthe switching transistor connected to the collector of the transistor 14at the output of the differential amplifier, while the emitter of theswitching transistor 25 is connected to the positive pole of the powersource 20 and the collector to the base of a transistor 27 through aresistance 26. The resistance 26 and the transistor 27 constitute acircuit for effecting scaleover of the pointer of the voltmeter 9. Theemitter and collector of the transistor 27 are connected between thevoltmeter 9 and the junction of the emitter of the transistor 7 and theresistance 8. The voltmeter 9 is so constructed that its pointer willscale over when the current supply is cut off. Thus arranged, the deviceoperates as follows.

When the object brightness is within the photometric range, theswitching transistor 25 stays conductive and hence the transistor 27 isalso conductive, while the voltmeter 9 is connected to theemitter-follower circuit 7, 8 by means of transistor 27, so that thevoltmeter indicates the proper shutter speed as shown in FIG. 2(a).

On the other hand, if the object brightness is lower than the lowerthreshold value of the photometric range, the switching transistor 25becomes non-conductive and hence the transistor 27 is alsononconductive, while the voltmeter 9 is disconnected from theemitter-follower circuit 7, 8 by the transistor 27, so that the pointer9a of the voltmeter 9 scale over as shown in FIG. 2(b), thus indicatingthat the object brightness is outside the photometric range.

The object of the present invention can be also attained by using anexposure indicator 9 of a fixed type unlike that in the describedembodiments. Also, although the present device has been described asadapted to a photometer in an electric shutter circuit, it can of coursebe applied to a photometer circuit for an automatic aperture controlledcamera. In the latter case, film sensitivity and shutter speed, asexposure factors, are conducted into the variable resistance 5 and thecontrol circuit 10 is arranged to control the objective lens opening,while giving aperture graduations on the scales of FIGS. 2(a) and 2(b).

In the foregoing description of the present invention, I discussed caseswherein the object brightness was below the lower limit of thephotometric range, but the same off-photometric-range indication can bemade similarly when the object brightness is over the upper limit of thephotometric range. Also, in the abovedescribed first and secondembodiments of the present invention, there was used a differentialamplifier as a component part, but such differential amplifier is not anindispensable part but may be substituted by other suitable means. Thepresent invention is also not restricted to the arrangement in which theswitching circuit is energized when the output from the photometriccircuit becomes lower than the output from the reference voltagegenerating circuit; it is possible to use any comparison circuit inwhich the outputs from the photometric circuit and from the referencevoltage generating circuit are received as inputs and the switchingcircuit is energized when the brightness of the object to bephotographed has run off the photometric range.

As apparent from the foregoing description, the off-photometric-rangeindication is effected in the present invention by using the indicatorof the photometer, so that there is required no specific means formaking such indication and hence no extra space therefore is needed.Further, since the indicator scales out when the object brightness isoff the photometric range, there is no possibility that the indicatorwill give any false indication for exposure. Battery exhaustion is alsominimized as the current consumption is small when the indicator remainsat its scaled-out position.

I believe that the construction and operation of my noveloff-photometric-range indicating device will now be understood and thatthe several advantages thereof will be fully appreciated by thosepersons skilled in the art.

I claim:
 1. In a device for indicating off-photometric-range of aphotometric circuit in an exposure indicator, the indicating deviceincluding:a photometric circuit .[.(1, 2).]. adapted to generate a firstvoltage in correspondence to the object brightness of an object to bephotographed; an exposure factor introduction circuit .[.(5, 6).].adapted to generate a second voltage which varies according to exposurefactors; and an indicator .[.(9) connected at one input terminal to saidphotometric circuit and at its other input terminal to said exposurefactor introduction circuit and.]. adapted to indicate a proper value ofexposure corresponding to the difference between said first and secondvoltages, the improvements comprising:a. a detection circuit .[.(11, 12,13, 13, 15, 16, 19).]. connected to said photometric circuit and adaptedto detect the object brightness and issue an output signal when thedetected value of the object brightness is off the photometric range ofsaid .[.indicator,.]. .Iadd.photometric circuit;.Iaddend. b. a switchingcircuit .[.(17, 25).]. connected to said detection circuit and arrangedto operate when receiving said output signal; and c. a circuit .[.(18,26, 27).]. connected between said switching circuit and said indicatorand actuated by operation of said switching circuit to scale out saidindicator thereby to give the off-photometric-range indication.
 2. Thedevice set forth in claim 1, wherein said detection circuit comprises:a.a reference voltage generating circuit .[.(11, 12).]. which generates afixed voltage equal to the output voltage produced by the photometriccircuit when the object brightness is at the limit of the photometricrange; and b. a comparison circuit .[.(13, 14, 15, 16, 19).]. whichreceives the outputs from said reference voltage generating circuit andfrom said photometric circuit as inputs and compares said inputs andissues a signal to actuate said switching circuit when the objectbrightness is detected to be off the photometric range.
 3. The deviceset forth in claim 2, wherein said comparison circuit is a differentialamplifier.
 4. The device set forth in claim 2, wherein said switchingcircuit, when made conductive, operates said scaling-out circuit.
 5. Thedevice set forth in claim 2, wherein said switching circuit, when madenon-conductive, operates said scaling-out circuit.
 6. The device setforth in claim 4, wherein said scaling-out circuit, when energized,generates a predetermined output voltage at one of the input terminalsof said indicator to scale-out said indicator.
 7. The device set forthin claim 5, wherein said scaling-out circuit, when energized, shuts offthe output at one of the input terminals of said indicator to cut offelectric current flowing to said indicator thereby to scale-out saidindicator.